In recent years, demand for secondary batteries has been rapidly increasing, because of advent of a large number of portable electronic appliances such as a camera-integrated VTR, a mobile phone and a laptop computer. With a tendency to miniaturization and lightweight of these electronic appliances, the secondary batteries have required a larger energy density intended for use as portable power sources. Among the secondary batteries, a lithium ion secondary battery is expectable in virtue of its energy density larger than that of conventional aqueous electrolyte secondary batteries such as a lead battery and a nickel-cadmium battery.
The lithium ion secondary battery practically uses, for a cathode active material, a lithium-cobalt composite oxide comprising a layer structure, a lithium-manganese composite oxide having a spinel structure, a lithium-nickel composite oxide or the like.
(1) The lithium-manganese composite oxide having the spinel structure exhibits a stabled crystal structure, whereas there are problems that a charge capacity of the lithium-manganese composite oxide is lower than that of the lithium-cobalt oxide or the lithium-nickel oxide, and a high temperature storage characteristic thereof is also somewhat inferior thereto.(2) The lithium-nickel composite oxide is expectable in virtue of excellence in material cost and stabilization of supplying superior to the lithium-cobalt composite oxide. However, there are problems that the lithium-nickel composite oxide has a crystal structure of low stability, and thereby causes lowering of charge/discharge capacity and energy density and/or degradation of charge/discharge cycle characteristics under high temperature environment.
As for methods for achieving stability of the crystal structure of the lithium-nickel composite oxide to suppress lowering of charge/discharge capacity and energy density, there are, for instance, proposals for methods such as one of upgrading cycle characteristics by substitution of a part of nickel with a different species of element as described in Japanese Patent Laid-open Nos. Hei 5-283076 and Hei 8-37007, one of using a specific metal salt or the like for an additive as described in Japanese Patent Laid-open No. Hei 7-192721 and one of regulating a binder in a process of synthesis of a cathode as described in Japanese Patent Laid-open No. Hei 10-302768. However, it is still necessary for the lithium-nickel composite oxide to have a more stabled crystal structure to meet recent requirements for larger density of electronic appliances or the like and higher speed of integrated circuits or the like, or environmental resistance required for mobile appliances or the like.
There is also a proposal for a method of adding, to the lithium-nickel composite oxide, the spinel-type lithium-manganese composite oxide whose crystal structure is stable. However, this method is disadvantageous in that the spinel-type lithium-manganese composite oxide has low charge capacity, and thereby causes lowering of charge/discharge capacity of a cathode without exploiting large capacity of the lithium-nickel composite oxide.
(3) The lithium-cobalt composite oxide is widely used, because cost and physical properties such as charge capacity and thermal stability are best balanced. However, the lithium-cobalt composite oxide has problems in cost and stabilization of supplying due to a small output of cobalt.
In the lithium-cobalt composite oxide being charged/discharged ranging from 4.250 V to 3.00 V to lithium metal results in a mean discharge voltage of 3.9 V to 4.0 V or around. Thus, an over-discharge state of a lithium ion battery with a cathode composed of the lithium-cobalt composite oxide increases a potential of an anode, and thereby causes problems such as dissolution of copper foil used for a current collector so as to exert a bad influence such as lowering of capacity upon the battery when the battery was recharged. Thus, the above lithium ion battery employs an external element such as a protection circuit to regulate a voltage in a final stage of discharge, and this constitution has been obstacles to miniaturization and cost reduction of the above lithium ion battery.
The present invention is conceived in view of the above conventional situations, and is to provide a cathode active material having large charge/discharge capacity and increased energy density of a cathode, and besides, being capable of achieving excellent charge/discharge cycle characteristics not only at room temperature but also under high temperature environment, and also to provide a non-aqueous electrolyte battery using the cathode active material.
In a lithium ion battery with a cathode composed of a lithium-cobalt composite oxide, the present invention is also to provide a cathode active material capable of realizing a lithium ion non-aqueous electrolyte secondary battery, which has a large capacity and is excellent in over-discharge resistance, and also to provide a lithium ion non-aqueous electrolyte secondary battery using the cathode active material.